"By the time the convention is over, the speakers will have used the phrase “middle class” much more than “working class.” But, last night, Michelle Obama used the phrase “working class” at least once, when she said Barack Obama “was raised by grandparents who were working class folks just like my parents, and by a single mother who struggled to pay the bills just like we did.” Earlier, she’d described her father as “a blue-collar city worker” and mentioned that her mother had stayed at home to raise her and her brother, Craig Robinson, now the men’s basketball coach at Oregon State.
Michelle Obama’s emphasis on her blue-collar roots served several purposes. It inoculates her and her husband against the Republican attacks that they’re out-of-touch elitists, in addition to Barack being a secret Muslim, Michelle being “bitter,” and both being anti-American radicals. By revealing her roots in the black working class--and rooting Barack Obama implicitly in the white working class--she refutes right-wing populist attacks that the Obamas look down on working Americans."
http://blogs.tnr.com/tnr/blogs/the_plank/archive/2008/08/25/michelle-obama-s-class.aspx